The silencer of a vehicle exhaust assembly may comprise an outer wall and porous exhaust pipe which defines an inner wall of an annular cavity. Typically, the cavity is randomly packed with sound absorbent material, for example, a glass fibre or basalt fibre material is positioned around the perforated pipe so as to fill the void between the pipe and the outer wall of the silencer and absorb sound waves which pass thereinto.
Since sound absorbent material in the form of glass fibre and basalt fibre is unpleasant to handle, suppliers of such fibrous materials supply the material to vehicle exhaust manufacturers in a suitably shaped and sized pack which is then inserted into the cavity of the silencer. Although the inner wall of the silencers are typically circular in cross section, the outer wall may be circular or oval and, increasingly, the oval outer wall is becoming more common. Numerous attempts have been made to improve the acoustic performance of silencers and it is one of the objects of the present invention to provide a silencer with a silencer cartridge with an improved acoustic performance.
A further problem with silencers of this type is that they are continuously subjected to directional gas flow which tends, over the course of time, to pack down the sound absorbent material or blow it out of the silencer altogether. Furthermore, the sound absorbent material is constantly vibrated so that the silencer material loosens over time and any structure present in the material collapses. The vibrational forces together with the uni-directional flow of gas through the silencer cause rapid degradation of the sound absorbent material leading to the necessity for its eventual replacement. It is a further object of the present invention to overcome these problems.
A still further problem with existing silencers, is that they are typically random packed with sound absorbent material. The random packing process tends to lead to a relatively wide variation in packing densities leading to difficulties in re-producibility and quality control in the final silencer product.
It is a still further object of the present invention to overcome this problem.
U.S. Pat. No. 5,670,756 discloses a silencer with the sound absorbing material wound directly around the outside of the inner sleeve with two or more turns so that the thickness of wound sound absorbing material around the inner sleeve is greater than the dimension of the gap between the inner sleeve and the outer sleeve to give an interference fit between the sound absorbing material and the inner and outer sleeves. The sound absorbing material with the inner sleeve is press fitted into the outer sleeve. The sound absorbing material is prevented from being blown out by exhaust gas flowing through the inside of the inner sleeve so that the silencer is improved in durability. This is provided by the press-fitting of the sound absorbent material which prevents vibration of the sound absorbing material during use. In addition, some stability in density is achieved by this technique compared with random packed silencers. However, although the vibrational problem is reduced, no attempt is made to solve the problem of the long term effect of uni-directional gas flow on the sound absorbing material.
U.S. Pat. No. 5,718,045 describes a similar technique where a sound absorbing material is formed of a heat-resistant fibre layer which is wound around an outer peripheral surface of the porous inner pipe with a binder inter-posed therebetween, and an outer peripheral surface of the heat-resistant fibre layer is press-fitted to an inner peripheral surface of the outer pipe with a predetermined interference. However, again, no attempt is made to address the problem of silencer degradation due to uni-directional gas flow.